THIS INVENTION relates to display devices, particularly, but not exclusively, to liquid crystal displays (LCDS) and is especially, but again not exclusively, of use in relation to back-lit LCD displays.
A liquid crystal display conventionally comprises, in the case of a passive super twisted nematic display, a liquid crystal cell mounted between light-polarising sheets, the cell comprising a pair of transparent, (e.g. glass) plates spaced apart to define therebetween a space which is filled with the liquid crystal medium, the opposing surfaces of the glass plates, i.e. the surfaces adjoining the liquid crystal medium, being provided with transparent (e.g. tin oxide) electrodes. The application of appropriate potentials to these electrodes by controlling circuitry produces localised electric fields in the liquid crystal medium which produce localised variations in polarisation of polarised light passing through the cell via one of the light polarising sheets and thus produce corresponding localised variations in intensity of light transmitted from the cell via the other light-polarising sheet, the latter variations taking the form of desired textual or numeric representations, or pixelated graphic or video displays, for example, dependent on the particular application of the display. Multi-colour displays are typically provided for by red, green and blue filters mounted over respective pixel areas of the display and located, for example, on the glass plate bearing the corresponding pixel electrodes.
An alternative form of liquid crystal display, the active matrix TFT (thin film transistor) LCD is similar in structure to that described above, except for the incorporation on the electrode-bearing plates, of active electronic (semi-conductor) components controlling the energisation of the individual electrodes, and except in some other respects which are of no relevance for the purpose of description of the present invention.
In back-lit LCDS, the source of the light directed through the liquid crystal cell or cells is a back-light assembly disposed behind the display. Front-lit LCDs are known, particularly as alpha-numeric displays, in which a reflector disposed behind the LCD serves to reflect ambient light reaching the reflector through the LCD back through the LCD to the viewer. (In front lit LCDs, assuming that polarisation is preserved by the reflector, only a single polarising sheet, on the outer side of the liquid crystal cell, may be required).
Super twisted nematic (STN) LCDs suffer from poor performance at wide viewing angles from the normal, (i.e. perpendicular) due to the optical characteristics of the liquid crystal materials. This is manifested in low contrast and colour inversion at high viewing angles. STN LCDs have also, in the past, suffered from slow response speed making the display technology inappropriate for live video. Recent technical developments such as dual scan addressing techniques have increased the response speed of STN LCD panels (i.e. displays) to levels very close to video rates.
Despite the better performance of the technically superior active matrix TFT LCDs, passive STN LCD panels are still used in many consumer products in preference, due to their significantly lower costs, and further technical developments and improvements are expected to increase this trend further.
Several techniques have been suggested for extending the useful viewing angles of back-lit LCD panels in general and those of STN LCD panels in particular. One such method, disclosed in xe2x80x9cViewing-angle-enhancement system for LCDsxe2x80x9d, S. Zimmerman et al. Journal of Society for Information Display, Volume 3, 1995, pp 173-176, involves the introduction of an optical element in the backlight assembly to reduce the divergence of the light incident on the LCD panel. A second optical element, acting as a diffuser, is then positioned on the outer surface of the outer polariser (i.e. that further from the back-light assembly), which diffuser acts to redistribute the light over the chosen viewing angle.
Collimating the light output of the backlight assembly, so that the maximum of light from the backlight assembly passes normally through the LCD, i.e. perpendicular to the major plane of the display, maximises the contrast potentially available, whilst the diffuser in front of the display allows the high contrast obtained to be perceived over a wider viewing angle than would be the case without the diffuser. (Generally, without such a diffuser, the perceived contrast and brightness fall off rapidly away from the normal, i.e. perpendicular, viewing angle).
Despite the good performance of the arrangement disclosed in the Zimmerman et al article referred to above, that arrangement, and in particular the provision of the second optical element or diffuser on the outer surface of the outer polariser, as described in that article, has several disadvantages and limitations. In that arrangement, the light is redistributed in the diffuser by an array of 3-D micromachined surface structures formed in micro-optical films and which employ Total Internal Reflection (TIR) light channelling. The surface relief micro-optical film, with a thickness of up to 400 xcexcm, needs to be positioned on the outer face of the outer polariser and cannot be sandwiched between the outer polariser and the liquid crystal cell for example. Bonding of such micro-optical film by means of adhesive to the outer polariser would reduce the effectiveness of the film as a diffuser because the adhesive would serve, to some degree as a refractive-index matching medium and would therefore reduce the effect of the surface relief features. Due to its fragility such a surface relief micro-optical film requires a further protective glass plate with an appreciable air gap and with additional anti-reflection coatings. Due to the regularity of the micro-optical elements there is high risk of Moire interference effects between the diffuser and the black mask of the LCD panel. The multitude of components coupled with the inflexibility and complexity of the optical design and manufacture result in a very expensive and bulky addition to what should, from the viewpoint of the consumer product manufacturer, be a low cost display.
It is among the objects of the present invention to provide an improved LCD display which avoids or reduces the above-noted disadvantages.
According to one aspect of the present invention, there is provided a display incorporating a screen comprising a light-polarising sheet and a polarisation-maintaining (non-birefringence) light diffusing sheet or layer disposed in front of or behind said light polarising sheet, and spaced therefrom, or in contact therewith or bonded thereto. Alternatively, the screen may comprise a light-polarising sheet which itself acts as a polarisation-maintaining diffuser, for example as a result of the relief optical features on a surface thereof or as a result of bulk features incorporated in the polarising sheet.
According to another aspect of the invention, there is provided a liquid crystal display of the kind comprising a liquid crystal cell or cells with a light-polarising sheet disposed in front of the liquid crystal cell or cells whereby the liquid crystal cell can be viewed through said light polarising sheet, characterised in that a polarisation-maintaining (non-birefringence) light diffusing sheet or layer is interposed between the liquid crystal cell and said light-polarising sheet or is disposed in front of the light polarising sheet.
According to another aspect of the invention there is provided a liquid crystal display of the kind comprising a liquid crystal cell or cells mounted between light-polarising sheets, the display being adapted to be viewed through one such light-polarising sheet, herein referred to as the front polarising sheet when illuminated through the other such light-polarising sheet, herein referred to as the back light-polarising sheet, characterised in that a polarisation-maintaining (non-birefringence) light-diffusing sheet or layer is interposed between the liquid crystal cell and the front polarising sheet.
According to still another aspect of the invention, there is provided a liquid crystal display of the kind comprising a liquid crystal cell or cells mounted between light-polarising sheets, the display being adapted to be viewed through one such light-polarising sheet, herein referred to as the front polarising sheet when illuminated through the other such light-polarising sheet, herein referred to as the back light-polarising sheet, characterised in that a polarisation maintaining (non-birefringence) light-diffusing sheet or layer is disposed on the front polarising sheet and is thus interposed between the front polarising sheet and the eye of the viewer.
In embodiments of the invention according to any of the above aspects, the diffuser may comprise a discrete sheet spaced from the outer polariser and/or from the liquid crystal cell, or may be bonded to the outer polariser and/or to the crystal cell or may form a coating on one or the other of these components.
It is a further object of the invention, in another of its aspects, to improve the efficiency of back-lit LCD displays.
According to this aspect of the invention, there is provided an LCD display of the kind comprising a liquid crystal cell interposed between a front light-polarising sheet or analyser and an arrangement for directing appropriately polarised light through said liquid crystal cell and said front polarising sheet or analyser, said arrangement including a light source, a diffuser arranged in front of the light source, and a reflective polariser arranged in front of the diffuser and thus disposed in a location between the liquid crystal cell and the diffuser, characterised in that the diffuser is a polarisation maintaining (non-birefringent) light-diffusing sheet or layer.
Preferably, said reflective polariser is a cholesteric filter arranged in front of the diffuser and a quarter wave plate is arranged in front of the cholesteric filter and thus disposed in a location between the liquid crystal cell and the cholesteric filter.
Alternatively, the reflective polariser may utilise the phenomenon of polarisation of light by reflection from smooth dielectric surfaces, classically investigated by Brewster, and may, for example, comprise a plurality of substantially planar interfaces between media of different refractive indices, inclined with respect to the direction of incidence, to the reflective polariser of light from said light source.
Preferably, in any of the above aspects, the polarisation-maintaining light-diffusing sheet or layer comprises a photopolymer incorporating graded refractive index optical features.
Preferably the graded refractive-index optical features are randomly distributed through the photopolymer.
According to yet another aspect of the invention, there is provided a light diffusing sheet or screen which is partly light reflecting and partly light-transmitting.
According to a still further aspect of the invention, there is provided an LCD display comprising a liquid crystal cell or cells with a light polarising sheet disposed in front of the liquid crystal cell or cells whereby the liquid crystal cell can be viewed through said light-polarising sheet, the display further comprising a screen according to the immediately preceding aspect disposed behind the liquid crystal cell and a light source disposed behind the partly light-reflecting, partly light transmitting screen, whereby the display can be viewed in ambient light, with said light source de-activated, so that ambient light passing through the liquid crystal cell to said partly reflective screen will be reflected thereby to return through the cell for viewing, and so that in low levels of ambient lighting, the light source can be activated to produce light to pass diffusely through said screen and through the liquid crystal cell and the polarising sheet, to the viewer.
According to a yet further aspect of the invention there is provided an LCD display comprising a liquid crystal cell or cells with a light polarising sheet disposed in front of the liquid crystal cell or cells whereby the liquid crystal cell can be viewed through said light-polarising sheet, the display further comprising a fully reflective, light diffusing sheet or screen disposed behind the liquid crystal cell whereby the display can be viewed in ambient light so that ambient light passing through the liquid crystal cell to said fully reflective, light diffusing screen will be reflected thereby to return through the cell for viewing.